Dysregulated signal transduction in immune cells is known to be associated with the development of various autoimmune and inflammatory diseases. Rheumatoid arthritis (RA) is an autoimmune disease that is characterized by persistent inflammation of the joints, which results in chronic tissue destruction. Many studies from animal models and RA patients have revealed an important role for CD4+ T cells in the development of RA. However, little is known about how inherited and acquired defects in T cell receptor (TCR) signaling translate into different outcomes such as autoimmunity, and which factors affect the activation and expansion of pathogenic self-reactive T cells. The broad objective of this proposal is to elucidate the roles of self-tolerance in the thymus and cytokine milieu in the synovium in the pathogenesis of RA. In this proposal, I will focus on two mouse variants, the SKG mice and YYAA mice, each with partial defects in TCR signaling due to mutation in ZAP-70, a tyrosine kinase crucial to TCR signaling. SKG mice and YYAA mice exhibit several similarities such as impaired T cell development, altered TCR signaling, and defective positive and negative selection. Despite the similarities, SKG mice have recently shown to develop spontaneous arthritis similar to human RA whereas severe arthritis was not found in the YYAA mice. To understand the mechanisms underlying the difference between SKG mice and YYAA mice, I proposed two specific aims. First, I will determine the impact of a graded change in thymic threshold on the development of autoimmune disease by crossing YYAA mice with SKG mice or with ZAP-70 null mice. Second, I will determine how homeostatic proliferation and cytokine production from autoreactive T cells contribute to the development of RA by comparing the cytokine profile of both mice and examining the proliferation index and the expression levels of pro-survival proteins in memory CD4+ T cells from both mice. These studies will help to understand how quantitative differences in TCR signaling will lead to the development of RA and may provide insights into mechanisms relevant to T cell autoimmunity that could lead to novel therapeutics. PUBLIC HEALTH RELEVANCE: The key features of RA are autoimmunity, chronic inflammation and joint destruction. Our studies of autoimmunity threshold might improve our understanding of how T cell tolerance is breached. In addition, studying how cytokines are organized within a hierarchical regulatory network should help identify important checkpoints that facilitate the progression from autoimmunity to chronic inflammation.